Fine structure, distribution and frequency of plasmodesmata and pits in the cortex ofLaminaria hyperborea andL. saccharina.

Prior to a long-distance transport of photoassimilate in the sieve elements ofLaminaria, a parenchyma transport across the cortex must occur. It is suggested that this transport is a symplastic one. The structural basis for this statement, continuous cytoplasmic interconnections of cells along the transport pathway, is demonstrated here forL. hyperborea andL. saccharina. The distribution, size, and frequency of pit fields in cell walls of all planes were determined. The data suggest that the conductivity for assimilate transport in the cortex is highest in the long axis of the thallus, not radially across the cortex. The fine structure, arrangement and number of plasmodesmata in pit fields were studied. The estimated flux rates and the anatomical findings clearly point to a symplastic parechyma transport of photoassimilate in the cortex ofLaminaria.